The present invention relates to change-speed transmissions and more particularly to an automatic transmission using a hydrokinetic unit, such as a torque converter, followed by a planetary gearing unit.
The conventional automatic transmission of the above type has the following advantages when used in an automobile:
(1) The hydrokinetic unit insures smooth and effortless start-up of the automobile;
(2) It insures shockless shifting even when the accelerator is kept depressed;
(3) It absorbs mechanical vibration derived from variations of torque of the engine; and
(4) It lengthens the life span of the associated planetary gearing unit.
However, the automatic transmission has the following disadvantages:
(1) It causes the associated engine to consume more fuel than if equipped with a manual transmission, and;
(2) It has a sluggish response to demand for engine braking.
In an attempt to alleviate these disadvantages, there has been proposed an automatic transmission having a hydraulic torque converter with at least one clutch and planetary gear set combination in which torque of the engine is split and transmitted through two flow paths. In this prior art automatic transmission, the direct drive or bypass clutch is selectively operable to interconnect the pump of the hydraulic torque converter and the turbine thereof for simultaneous rotation therewith to thereby bypass the torque converter and eliminate slippage in the transmission. In other words, the torque can be transmitted either totally through the torque converter, or directly, but nothing in between. In a second prior art transmission, a gearing unit has a rotary element connected to the turbine of the torque converter, and the pump is connected to another rotary element so that the input torque of the engine is at all times transmitted through two flow paths, one including the torque converter, and the other including the other rotary element. In this arrangement, the hydrokinetic unit cannot be totally bypassed.
A disadvantage with the first prior art automatic transmission mentioned above resides in that the fuel economy during all low and middle speed operating is still compromised. The economy is bad because under these operations conditions, the direct clutch is disengaged and the hydraulic torque converter delivers the full torque from the engine to the gearing unit. Another disadvantage is that over a given range of speed of rotation, the variation of torque of the engine is great. This is especially undesirable during the middle and relatively high speed ranges after the direct clutch has been engaged. As the automobile slows down in these ranges, the engine tends to start "lugging" causing a rough driving condition and undue wear on the engine. In the second mentioned prior art transmission, the shortcoming arises because overall operating conditions, ranging from low speed operation to high speed operation, a portion of torque is always transmitted from the engine to the gearing unit through the torque converter. Thus, the overall fuel economy leaves much to be desired due to the slip of the torque converter in the high speed operation.